The present inversion relates to an improved sensor for detecting localized pressure, for a wide variety of applications.
Sensors for the detection of localized pressure (i.e. pressure imposed on a particular object as distinct from atmospheric pressure) may employ the principle whereby the intensity of light or other wave energy from a source, which is diffused and scattered within a scattering medium such as translucent foam, is increased in the vicinity of the light source as the concentration of scattering centers within the medium increases, i.e., the average distance between scattering centers decreases. A similar effect is achieved if the nature of the scattering centers changes to change their light reflective or refractive properties.
The region within the medium which contains scattered light from the source is known as a xe2x80x9cvirtual optical cavityxe2x80x9d since this region effectively emulates an optical cavity. For simplicity, this region will be referred to as simply an xe2x80x9coptical cavityxe2x80x9d. The intensity of diffused or sattered light at any particular position within an optical cavity is referred to as the xe2x80x9cintegrated intensityxe2x80x9d of the light at that position. Thus, as the medium is compressed by the application of pressure, the integrated intensity of the light within the region immediately surrounding the light source increases in intensity. The increase is proportional to the increase in concentration of scattering centers or a change In the nature of the scattering centers. This in turn may be related to increases in localized pressure applied to the medium. The consequent decrease in light intensity occurs within a more distant region within the medium. For example, U.S. patent application Ser. No. 08/895,268 (Reimer et al.) describes a pressure sensor based on this principle, in which the scattering medium may comprise either a material having scattering centers dispersed generally evenly therein, or a hollow deformable container, the inner surface of which diffuses light or other wavelike energy directed into the medium. The light source forms an integrated cavity within the medium, defined by a region containing fully scattered light from the source. When pressure is applied to the medium, the medium compresses and increases the concentration of scattering centers in the region surrounding the light source. The resulting increase in light intensity is detected by a receptor and communicated to an information processor. In one version, a multiplicity of light sources and receivers permits the general location of the pressure to be resolved. Within an apparatus of this type, one or more light sources and detectors are provided, with each source and a corresponding detector being generally adjacent to each other or dose together. Most conveniently, the scattering medium comprises a compressible, translucent material such as plastic foam. An array of source/detectors pairs may be provided to provide localized pressure detection means. The detector or detectors are associated with a signal processing unit, which receives information from the detectors corresponding to the detected integrated light intensity levels, and resolves this information into a corresponding pressure level experienced by the scattering medium.
References herein to the word xe2x80x9clightxe2x80x9d includes within its scope light in visible and non-visible wavelengths.
The pressure sensors of the type characterized within the above-referenced prior art, may be subject to xe2x80x9cnoisexe2x80x9d as a result of several factors. Most importantly, interference may result in a change in the light absorption characteristics of the scattering medium, or the scattering centers themselves. A change in absorption would affect light intensity within the regions surrounding the light source, and this could be mistaken for a deformation effect Such a change might take place in a polymeric medium as the result of long term aging photo-oxidation. It would therefore be valuable to provide more robustness to this type of sensor by enabling it to better differentiate noise from signal. It is accordingly desirable to integrate within a sensor of this sort, a means to measure the light absorption properties of the scattering medium. Absorption measurement for optical energy or other forms of directly transmitted or reflected wave energy is a well known art, and the principles for the measurement for absorption in transmission or reflection by means of various photometers have been thoroughly documented in scientific literature. However, it has not been previously proposed to introduce an absorption measuring element into a pressure sensor of the above type.
An object of the invention is to provide a pressure sensor for detecting point source or localized pressure, which operates by detection of the intensity of scattered light within a compressible carrier medium, and which has an improved signal to noise ratio.
In one aspect, the invention comprises a pressure sensor having improved signal to noise sensitivity, of the type comprising a volume of a generally translucent material having light scattering centers evenly dispersed therein, and which is readily deformable under pressure;
a source of light or other wave energy associated with the material and positioned to direct light into the material to form a virtual optical cavity within the material within which light from the source is fully scattered;
a first light detector, in operative association with the translucent material positioned generally adjacent to or in the immediate vicinity of the light source, for detecting light intensity within the optical cavity; characterized by:
a second light detector in operative association with the translucent material, for detecting the intensity of light within the material at a position outside the optical cavity; and
signal processing means operatively associated with the two detectors, for receiving light intensity information from the detectors, and resolving the information thus received into a measure of localized pressure bearing on the material.
In the above aspect, the invention takes advantage of the phenomenon whereby deformation or compression of a material having light scattering centers dispersed therein causes an increase in the integrated intensity of light emanating from a point source within an optical cavity formed by the light within the material. The increase is proportional to a decrease in light intensity within the material at positions immediately outside, the optical cavity. By measuring scattered, or integrated, light intensity both within the optical cavity and immediately outside the cavity, an enhanced sensitivity and improved ability to discriminate changes in light scattering from changes in absorption, may be achieved over measurement only of light intensity within the cavity. The results apply both with respect to measurement and for purposes of noise signal discrimination.